I recently bought a new laptop, specifically a Fujitsu P7120 (based in large part on Joey's plug for it). I'm generally happy with it, but I haven't been able to get the headphone jack to work.
Today I had a few minutes and was fiddling around with it, when I noticed that there was an extra column in alsamixer labelled "headphone jack mode". This isn't a volume bar; it just displayed the text "50". Just on a whim, I tried hitting "up" on it, and quickly cycled through "80", "Line In", and then "Line Out". Jackpot! All of a sudden, sound started piping out of the headphones.
So, just in case anyone else is puzzling over this problem, make sure you've checked ALL the knobs alsa lets you turn!
I still wonder why alsa is defaulting to using a headphone jack as a microphone input...but I'm not so curious to, say, stay up even later trying to figure it out.